The Blu-ray Disc™ standard using a blue-violet semiconductor laser currently enables commercialization of optical discs for consumer use that have a recording capacity of approximately 50 GB. Henceforth, it is desired to increase the capacity of optical discs up to approximately the same as the capacity of a hard disk drive (HDD) such as 100 GB to 1 TB.
In order to realize such an ultra-high density for optical discs, however, there is necessary a new densification technology that is different from the densification technology of shortening wavelength and increasing the NA of an object lens.
While studies related to a next generation storage technology are under way, there is a focus on the hologram recording technology that records digital information by using holography.
The hologram recording technology is a technology of recording information on a recording medium by superposing signal light that is two-dimensionally modulated by a spatial light modulator and includes page data information with reference light inside a recording medium and generating refractive index modulation inside the recording medium with an interference fringe pattern that occurs at the time of superposition of the signal light and the reference light.
When the recording medium is irradiated at the time of reproducing information with the reference light that is used in recording, the hologram that is recorded on the recording medium acts as a diffraction grating and generates diffracted light. The diffracted light is reproduced, including the recorded signal light and phase information as one light.
The reproduced signal light is two-dimensionally detected at high speed by using a light detector such as a CMOS and a CCD. As such, the hologram recording technology enables instant recording of two-dimensional information on an optical recording medium by using one hologram and further enables reproducing of the information. The hologram recording technology also enables writing of multiple pieces of page data at a certain location on the recording medium in a superposed manner. Therefore, it is possible to achieve recording and reproducing of information on a large scale and at high speed.
JP-A-2004-272268 (PTL 1) is an example of the hologram recording technology. What is disclosed in this publication is a so-called angle-multiplexed recording scheme. In the angle-multiplexed recording scheme, fluxes of the signal light are concentrated on an optical information recording medium by a lens, and concurrently, the optical information recording medium is irradiated with parallel fluxes of the reference light. Then the signal light interferes with the reference light, and a hologram is recorded. Furthermore, while the angle of incidence of the reference light on the optical recording medium is changed, different pieces of page data are displayed on a spatial light modulator and are recorded in a multiplexed manner.
In addition, the subject matter disclosed in US2007/0091399A (PTL 2) is intended to stabilize the angle-multiplexed recording and reduce noise in a manner of changing the phase of the signal light on a pixel basis by interposing a phase-modulating optical element (irradiance-tailoring element, referred to as a phase mask in the present specification hereinafter) on the optical path of the signal light and driving the optical element linearly or rotationally.